The invention relates to a servomotor for assisting in the operation, of a motor vehicle brake circuit.
The prior art of which in U.S. Pat. No. 3,109,287 is typical, relates to a servomotor for providing a power assist in the operation of a brake system. The servomotor comprising, in a housing, a power-assisting part and a part forming a hydraulic master cylinder. The power-assisting part being adapted in use to cooperate with an external power source to generate a pressure difference across a driving piston. The pressure difference being controlled by first valve means operated by an intake controlling element. The master-cylinder part comprising a chamber communicating with a fluid replenishing orifice and an outlet orifice. An assembly of two coaxial pistons comprising an annular piston attached to the driving piston and a central piston movable solely by way of the intake controlling element. The first valve means, when actuated from its release or idle position, cause the driving and annular pistons to move in unison. During a first phase of servomotor operation the replenishing orifice is disconnected from the chamber to permit the two coaxial pistons to generate a first pressure level in the chamber.
In devices of this type, when the maximum possible pressure difference is applied to the driving piston, further force transmitted by the operator enables the central piston to be moved towards the interior of the chamber. To increase the pressure at the outlet orifice, however, the operator must overcome a considerable reaction force which acts both on the central piston and on the annular piston, which projects into the chamber and is linked to the central piston by a spring.